This invention relates generally to an apparatus and a method for removing soluble portions of a coating to produce a high resolution image in the coating. More particularly, this invention relates to an apparatus and a method in which reciprocatory point pressure is applied to the surface of a coating to aid in the dissolution and physical removal of soluble portions of the coating. In one particularly important embodiment of this invention, an apparatus and a method are described for developing high resolution relief images on photopolymer coated plates for letter-press printing applications.
The following description of the invention, including the prior art and the preferred embodiments, has been confined to the development of photosensitive graphic arts articles, particularly photopolymer printing plates. The description has been so limited, however, only to facilitate an understanding of the inventon, and is not intended to limit the range of application or protection accorded the invention. Indeed, it will be clear to those skilled in the art from the discussion of the invention in the context of photopolymer printing plates that it will have wide utility in a broad range of applications where portions of coatings are to be removed from a substrate.
Modern letter-press printing now commonly utilizes printing plates having photographically produced relief images. These printing plates include a laminated structure consisting of a thin coating of a photosensitive composition bonded to a metal or plastic substrate. The desired relief image is obtained by exposing the plate through a photographic negative to create a differential in solubility in the photosensitive composition thereby producing a latent image corresponding to the desired relief image.
This latent image is developed into a relief image by dissolving and physically removing the portions of the photosensitive composition which are soluble in a properly chosen developer solution. One particularly suitable photosensitive material is the photopolymer described in U.S. Pat. No. 3,801,328 to Takamoto et al. This photopolymer may be developed with water.
Although numerous developing solutions are available for the various photopolymers described in the art, the range of known commercially practical techniques available for physically removing soluble photopolymer is limited to two: (1) mechanically abrading or rubbing the photopolymer coating with a developer solution laden sponge, cloth or brush; and (2) subjecting the photopolymer coating to a high pressure stream of developer solution. Each of these approaches to developing the photopolymer printing plate has serious drawbacks which are obviated by the present invention.
Conventional rubbing techniques for developing photopolymer plates entail vigorously rubbing a sponge, cloth, brush or other mildly abrasive article across the surface of the plate, typically in a circular motion, to accelerate dissolution and physical removal of the soluble photopolymer. Unfortunately, even with vigorous rubbing action, it may not be possible to clear the solubilized polymer out of the many recesses and crevices of the relief image. Indeed, this technique often not only fails to clear these areas, but also tends to carry earlier removed photopolymer particles back into highlight areas, and to abrade delicate insoluble portions of the image. As a result, letter-press prints produced from plates developed in this way lack the consistent high resolution qualities demanded in the marketplace.
Spray development techniques were devised to overcome some of the inherent drawbacks of the rubbing development techniques by both accelerating the development process and improving image resolution. In spray development, typically carried out in an apparatus 2 like that illustrated in FIG. 1, the developing solution 3 is directed against the surface of a printing plate 4 moving along conveyor belt 6 through high pressure nozzles 8 which typically reciprocate in a plane back and forth across the surface of the plate. The high pressure spray drives the developing solution into the photopolymer to help dissolve the solubilized portions of the latent image and to physically remove these portions from the printing plate.
Although certainly an improvement over rubbing development, spray development techniques have a number of their own unique problems. For example, the high pressure spray may damage delicate highlight portions of the insoluble image and soften portions of the image, complicating later drying steps. Also, the high pressure sprays produce a fine mist which contaminates the air surrounding the nozzles and results in loss of the developing solution. In addition, large quantities of developing solution must be stored in bulky reservoirs ready to be drawn through high pressure pumps (indicated at 10 in FIG. 1) for injection through the spray nozzles. A very significant problem inherent in the spray development approach is air entrainment or foaming 12 which occurs at the plate surface as the spray impacts against it. This foaming problem is aggravated by the photopolymer carried by the developer solution which encourages further air entrainment. If this foam is permitted to reach the high pressure pumps, uneven spraying and damage to the pumps due to cavitation will result.
In order to minimize foaming problems, it is necessary to maintain the lowest practicable level of photopolymer solids in the developer reservoir and to introduce defoaming agents. These defoamers are not only an unwelcome significant additional expense, they may interfere with the operation of the developer solution in dissolving the photopolymer.